1. Field of the Invention
The invention relates to thermal control of gas turbine engine cases and particularly for thermal control of clearances between turbine rotors and surrounding shrouds by counterflowing air used for heat transfer around the engine.
2. Description of Related Art
Rotor clearance control systems that incorporate heating and cooling to effect thermal control of shrinkage and expansion of different parts of gas turbine engine cases are used for aircraft gas turbine engines to reduce leakage losses and improve specific fuel (SFC) consumption of the engines. One example of such an apparatus can be found described in U.S. Pat. No. 4,826,397, entitled "Stator Assembly for a Gas Turbine Engine", by Paul S. Shook and Daniel E. Kane. Reference may be had to this patent, by Shook et al, for background information and, for this reason, it is incorporated herein by reference. Shook discloses a clearance control system that uses spray tubes that spray air ducted from the engine's fan or compressor to cool turbine engine case rings in order to thermally control the clearance between an engine turbine rotor section and a corresponding stator section shroud disposed around the turbine rotor section. The Shook patent attempts to control circumferential thermal gradients around the rings, or rails as they are referred to in the patent, by shielding and insulating the rails. The shielding does not eliminate the circumferential gradient but does reduce the magnitude and severity of the gradient and therefore the stress and clearance variation that such a severe circumferential thermal gradient causes.
However spray tubes behave as heat exchangers and a circumferential variation in the temperature of the heat transfer fluid cannot be avoided nor the attendant problems associated with such a circumferential variation as shown in the prior art. The circumferential variation in the temperature of the air used to thermally control the rings produces unequal expansion and contraction of the rings particularly during transient operation of the engine such as during take-off.
The circumferential temperature variation produces a mechanical distortion of the engine casing or rings associated with the casing commonly referred to as an out of round condition. Such out of round conditions further leads to increased rubbing of the rotor and its corresponding stator assemblies such as between rotor blades and surrounding stator shrouds or between rotating and static seal assemblies. The out of round condition causes increased operating clearances, reduced engine performance, a deteriorating engine performance, and reduced component efficiency. Often difficult and expensive machining of circumferential variations in the static parts is employed during the manufacturing of the casing components to compensate for the operational circumferential variations in the thermal control air.
Another example of a clearance control system is found described in U.S. Pat. No. 4,363,599, entitled "Clearance Control", by Larry D. Cline et al assigned to General Electric the assignee of the present invention, that discloses the use of control rings integrated into the turbine casing and supporting a turbine shroud that surround and seals about turbine rotor blades. Thermal control air is supplied to the rings to effect thermally induced clearance control between the turbine blade tips and the surrounding shroud. Thermal control air is supplied to the rings from an area surrounding the combustor and through axial extending passages in the casing and through the rings.
A General Electric CF6-80C2 turbofan gas turbine engine incorporates a case flange assembly as depicted in FIGS. 6, 6a, and 6b, labelled as prior art, having a turbine shroud thermal control ring 220 bolted between a compressor case flange 210 and a turbine case flange 216. Compressor flange 210 and turbine flange 216 have compressor and turbine flange cooling air grooves 260a and 260b respectively facing thermal control ring 220. Cooling air is fed into compressor flange cooling air groove 260a through a radial inlet slot 270a which is cut through compressor flange 210 to groove 260a.
Compressor and turbine flanges have bolt holes 226 which snugly receive bolts 240. Control ring 220 has alternating bolt holes 226 and enlarged bolts holes 230 that provides a cooling air passage through control ring 220 to turbine flange cooling air groove 260b. Radial cooling air exhaust slots 270b provide an exit for the cooling air from the flange assembly.
There are 34 bolt holes around the engine flange assembly and 17 sets of radial slots providing cooling air passages for thermal control around the ring. Cooling air is fed to the grooves at different circumferential locations thereby subject to circumferential variations in the cooling air temperature.